Nowadays, our living environment includes various kinds of electronic devices, such as a personal computer, a DVD player, and a video camera. Some of the electronic devices are connected to a power supply such as a battery or a wall outlet to operate, using a connection device such as a USB device. The connection device used in this case has a power source connection circuit for connecting an electronic device with a power supply.
The power source connection circuit is provided with a switch between an input terminal to which a power supply is connected and an output terminal to which an electronic device is connected, so that when an input voltage is an overvoltage or when a current passing through the switch is an overcurrent, the switch is turned off to prevent the overvoltage or the overcurrent from being transmitted to the output terminal.
In addition, since the switch may be destroyed when dirt or the like between the output terminal and the ground causes a short circuit and a gate-source voltage of the switch becomes an overvoltage, the power source connection circuit needs to discharge electric charges accumulated in the gate of the switch.
FIG. 1 is a circuit diagram of a conventional power source connection circuit 1. The power source connection circuit 1 includes an input terminal 3 which is connected to a power supply, an output terminal 5 which is connected to an electronic device, and a switch SW1 having its ends connected to the input terminal 3 and the output terminal 5. A drain D1 of the switch SW1 is connected to the input terminal 3, and a source S1 of the switch SW1 is connected to the output terminal 5. A gate G1 of the switch SW1 is connected to a step-up circuit CP for supplying electric charges to the switch SW1. Further, between the gate G1 of the switch SW1 and the source S1 of the switch SW1 is connected a Zener diode T1 for discharging electric charges accumulated in the gate G1. More specifically, the gate G1 of the switch SW1 is connected to a cathode K1 of the Zener diode T1, and an anode A1 of the Zener diode T1 is connected to the source S1 of the switch SW1.
A description will be given of the operation of the power source connection circuit 1. First, the step-up circuit CP supplies electric charges to the gate G1 of the switch SW1 and increases the gate voltage of the switch SW1 to turn on the switch SW1. Then, through the switch SW1, an input voltage VIN from the input terminal 3 is outputted from the output terminal 5 to an electronic device as an output voltage VOUT. When the output terminal 5 is short-circuited to the ground and a gate-source voltage VGS of the switch SW1 exceeds a breakdown voltage VD of the Zener diode T1, the Zener diode T1 is turned on to discharge the electric charges accumulated in the gate G1. A power source connection circuit having the same feature as the power source connection circuit 1 is disclosed, for example, in PTL 1.
FIG. 2 is a circuit diagram of another conventional power source connection circuit 21. The power source connection circuit 21 is different from the power source connection circuit 1 in that Zener diodes T1 and T2 are connected in series in opposite directions to each other between a gate and a source of a switch SW1. More specifically, a source S1 of the switch SW1 is connected to an anode A1 of the Zener diode T1, a cathode K1 of the Zener diode T1 is connected to a cathode K2 of the Zener diode T2, and an anode A2 of the Zener diode T2 is connected to a gate G1 of the switch SW1.
A description will be given of the operation of the power source connection circuit 21. First, a step-up circuit CP supplies electric charges to the gate G1 of the switch SW1 and increases a gate voltage of the switch SW1 to turn on the switch SW1. Then, through the switch SW1, an input voltage VIN from an input terminal 3 is outputted from an output terminal 5 to an electronic device as an output voltage VOUT. When the output terminal 5 is short-circuited to the ground and a gate-source voltage VGS of the switch SW1 exceeds a total voltage of a breakdown voltage VD of the Zener diode T1 and a threshold voltage Vf (a forward voltage Vf) of the Zener diode T2, the Zener diodes T1 and T2 are turned on to discharge the electric charges accumulated in the gate G1. A power source connection circuit having the same feature as the power source connection circuit 21 is disclosed, for example, in PTL 2.